Optical defect detection technology has been the bread and butter for the detection of various kinds of defects in semiconductor wafers. It has provided both high performance and high throughput, which other technologies like electron beam microscopy could not offer. However, as the design rules of IC chips decreased, it became harder to detect defects reliably. Especially, design rules of future generations of IC chips are so small that there is a real possibility that none of the current optical defect detection technologies work. Therefore, in order to extend the life of optical technology into future generation systems of defect detection, a major overhaul of optical defect detection technology is needed.
Known optical defect detection systems include both bright field systems and dark field systems. Unlike bright field systems, dark field systems attempt to exclude the unscattered beam from the image. However, limitations of the current dark field and bright field defect detection systems exist which cause difficulty in accurately detecting defects especially as the design rules progressively decrease. Separate path interferometric techniques have been proposed according to which two beams, probe and reference beams, are generated using a beam splitter and brought to an image sensor through different paths or subsystems. For example, separate path systems designed for defect detection are described in U.S. Pat. Nos. 7,061,625, 7,095,507, 7,209,239 and 7,259,869. Another separate path system which is designed for high resolution surface profiling is Linnik interferometer (see, M. Francon, “Optical Interferometry,” Academic Press, New York and London, 1966, p 289.) These separate path interferometric systems are, in principle, capable of amplifying the defect signal or measuring both the amplitude and phase of the defect signal. However, these systems are not only complex and expensive but also have a very critical drawback; they are unstable due to the two different paths the probe and reference beams take. Small environmental perturbations like floor vibrations, acoustic disturbance, temperature gradient, etc., can easily destabilize the system. Consequently, it is not only hard to build but also difficult to use this kind of separate path interferometric systems in industrial environments.
Conventional phase-contrast microscopes are designed to provide a fixed amount of phase control to specular component, usually π/2 or −π/2. These systems commonly use extended light sources such as an arc or halogen lamp. Although they are generally suitable for observing biological samples, conventional phase-contrast microscopes are not generally well suited for detecting a wide variety of defects that exist in semiconductor wafers and/or reticles.
U.S. Pat. No. 7,365,858. and U.S. Application Publication No. 2005/0105097 A1 describe a system for imaging biological samples. Two modes of operation are described, a “phase mode” and an “amplitude mode.” The goal in the described amplitude mode is to obtain high contrast raw images. In phase mode, the described techniques attempt to extract phase information only. The descriptions mention liquid crystal spatial light modulation which is performed in a pupil conjugate through the use of beam splitters and additional lens groups, which are prone to power losses.